1. Field of the Invention
The present invention relates to a method for managing a DSS (Digital Slow Shutter) mode in a monitoring camera, and more particularly to a method for normally outputting a monitoring video frame in a DSS mode.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating the configuration of a conventional monitoring camera to which a method for managing a DSS (Digital Slow Shutter) mode is applied. The conventional monitoring camera includes a lens 10, an iris 11, a CCD (Charge Coupled Device) 12, an A/D (Analog-to-Digital) converter 13, a DSP (Digital Signal Processor) 14, a memory (buffer) 15 and a microcomputer 16. The lens 10 can manually or automatically perform a focusing function and/or a zooming function. The A/D converter 13 converts video signals inputted from the CCD 12 into digital video data. The DSP 14 generates a video frame from the digital video data. The microcomputer 16 controls operations of the CCD 12, the A/D converter 13, the DSP 14 and the memory 15.
Because an amount of light incoming through the lens 10 is too small when the monitoring camera operates in a dark place where the luminance is very low, the microcomputer 16 performs an iris control operation for gradually opening the iris 11 so that an amount of incident light can increase. Moreover, the microcomputer 16 performs a CCD shutter speed control operation for gradually reducing the shutter speed of the CCD 12 so that an electrical signal level based on photoelectric conversion of the CCD 12 can be set to a certain level appropriate for signal processing.
When the iris 11 is maximally opened and simultaneously the shutter speed of the CCD 12 is minimum while the control operation is carried out, the microcomputer 16 automatically sets the DSS mode.
FIG. 2 is a flowchart illustrating the method for managing the DSS mode in the conventional monitoring camera. The monitoring camera installed in an arbitrary place performs a series of operations for outputting a video frame captured in a predetermined cycle, for example, a 1/30-second cycle, as a monitoring video frame (S10). As described above, when an amount of light incoming through the lens 10 is small, the microcomputer 16 performs an iris control operation for gradually opening the iris 11 and a CCD shutter speed control operation for gradually reducing the shutter speed of the CCD 12.
When the iris 11 is maximally opened (S11) and simultaneously, the shutter speed of the CCD 12 is minimum (S12) through the above-descried operation, the microcomputer 16 automatically sets the DSS mode (S13). In the DSS mode, the microcomputer 16 generates an output video frame by accumulating digital video data of consecutive video frames so that the output video frame can have a predetermined luminance or more.
The microcomputer 16 controls the A/D converter 13 and the DSP 14 to convert a weak video signal from the CCD 12 into digital video data and to generate a video frame by accumulating the digital video data, and temporarily stores the generated video frame in the memory 15 (S14). At this point, the microcomputer 16 checks the luminance of the temporarily stored video frame (S15).
When the luminance of the temporarily stored video frame is lower than a preset reference luminance (corresponding to a video frame recognizable by a user) as a result of the check (S16), the temporarily stored video frame is not outputted. As digital video data of the video frame temporarily stored in the memory 15 and digital video data of a newly inputted video frame are accumulated, an operation for generating a new video frame is continued.
On the other hand, when the luminance of the video frame temporarily stored in the memory 15 is equal to or greater than the preset reference luminance as the result of the check (S16), the video frame temporarily stored in the memory 15 is output as a monitoring video frame and then is deleted (S17). Until a system operation is terminated, the above steps (S14-S17) are iterated.
Accordingly, even though the monitoring camera operates in a dark place where the luminance is very low, the DSS mode operates, such that a monitoring video frame based on a luminance level recognizable by the user can be outputted.
However, because the conventional monitoring camera accumulates digital video data and continuously performs a temporarily storing operation without outputting a video frame until the luminance of a video frame temporarily stored in the memory is equal to or greater than a preset reference in the DSS mode, a monitoring video frame to be outputted in a predetermined cycle is cut off and delayed. For this reason, there is a problem in that a monitoring function is degraded.